1. Technical Field
This invention relates generally to battery charging and protection circuits, and more specifically to a thermally-limited charging circuit with overcharge and undercharge protection.
2. Background Art
Electronic devices, including cellular phones, pagers, radios, compact disc players, MP3 players, portable computers, and the like are becoming ever more popular. These devices are gaining popularity due to their portability. The devices derive their portability from the use of rechargeable batteries as a power source. Rechargeable batteries, of course, require a battery charger to inject current or xe2x80x9cchargexe2x80x9d, thereby causing the battery to store energy for future use in the electronic device.
FIG. 1 illustrates a simple battery charger 100 that is well known in the art. The charger 100 consists of a power supply 101, a linear regulator 102, a pass element 103 and a battery cell 104. The power supply 101 provides voltage and current to the battery cell 104. The voltage and current must be regulated by the pass element 103 so as to avoid charging the battery cell 104 too, rapidly. The linear regulator 102 performs this regulation by dissipating as heat the difference between the power generated by the power supply 101 and the power stored by the battery cell 104.
The problem with this prior art solution is that the pass element 103 can overheat. This is best explained by way of example. For a typical single-cell, lithium battery application, a fully charged battery cell 104 typically registers about 4.1 volts. Thus, to fully charge the battery cell 104, and to give enough headroom for parasitic power losses in the pass element 103 and connecting circuitry, the power supply must be capable of supplying at least 5 volts. A typical battery cell 104 will charge optimally at a current of roughly 1 amp.
The problem arises with the battery cell 104 is fully discharged. A discharged battery cell 104 may register only 2 volts. As the power supply 101 would supply energy at a rate of 5 volts at 1 amp, or 5 watts, and the battery cell 104 stores energy at a rate of 2 volts at 1 amp, or 2 watts, the pass element 103 must dissipate energy at a rate of 3 watts. As typical pass elements 103 may come in an industry-common TO-220 package, 3 watts for extended periods of time may make the pass element 103 quite warm. Extended periods of heat my actually jeopardize reliability by approachingxe2x80x94or surpassingxe2x80x94the threshold junction temperature of the pass element 103.
The problem is exacerbated when an incompatible power supply 101 is coupled to the circuit. For example, if someone accidentally couples a 12-volt supply to the charger, the pass element 103 may have to dissipate 10 watts! This can eventually lead to thermal destruction of the pass element 103.
One solution to this problem is recited in U.S. Pat. No. 5,815,382, issued to Saint-Pierre et al. entitled xe2x80x9cTracking Circuit for Power Supply Output Controlxe2x80x9d. This solution provides a means of reducing the output voltage of a power supply when the battery is in a discharged state, thereby reducing the total output power of the power supply. This, in turn, reduces the amount of power a pass element would need to dissipate.
While this is a very effective solution to the problem, it requires a power supply that both includes a feedback input and is responsive to the input by changing the output voltage. The electronics associated with an adjustable power supply can be more expensive that those found is a simple linear transformer power supply.
There is thus a need for an improved means of regulating temperature in a power-dissipating element like those employed as pass elements in battery charging applications.